Support for document notes

[evanmiller]

Stata has a "notes" feature which appears as a characteristic / expansion field in DTA files, though it is not well-documented.

SPSS files have an optional document record which allows writing an arbitrary number of 80-byte lines into the file.

It would be nice to provide an abstraction over these document features for reading and writing notes that accompany a data set.

[evanmiller]

Preliminary API:

// Read API
typedef int (*readstat_note_handler)(int note_index, const char *note, void *ctx);
readstat_error_t readstat_set_note_handler(readstat_parser_t *parser,
    readstat_note_handler note_handler);

// Write API
void readstat_add_note(readstat_writer_t *writer, const char *note);

Note that the index values passed to the note handler will be unique but not necessarily sequential -- it is the note number recorded in the DTA file.

I have an implementation that seems to be working for DTA files, and plan to add support for SPSS files later today.

[evanmiller]

Implemented with these commits:

DTA: 44eff95

SAV: ef45b7d

POR: 411bcbf

The round-trip tests all pass but it is possible that older DTA files won't be parsed properly -- the format of notes is poorly documented so I did some guesswork. Please file a new issue if data set notes are not appearing as expected.